We have an introduction/safety/warmup speech for all groups at the start of our attraction. I use a stopwatch to track time between groups, and will vamp on the speech if I need to stretch out the time. Depending on the group, I'll sometimes add in extra time between groups if it looks like it will be needed. Examples would be groups that have someone who can't walk very quickly, or a group of screaming high school girls that will take forever to get through the areas. Standard timing is about 3.5 minutes or longer on slow nights, 3 minutes on busy nights. Any faster and groups are running on top of each other. Much slower and it's really hard to keep the intensity up.

After a night or two, the stopwatch isn't really necessary, it becomes automatic.

On some individual scenes they'll have someone man the door to pace groups into the scene. Typically that's only used if some major resetting is needed.

we use a siren and a light to let the person manning the door know when to send in the next group. Once the group reaches a specific point in the haunt, that room sets off a siren for a second or two letting the first room know that the guests have cleared their room. its up to the worker in the first room to flash the light to signal the ok for the next group. This has worked very well for us for years. It guarantees that the groups have gotten far enough into the haunt and that the first room is ready for the next group before they are let in. Every once in a while there will be a mis-cue but it is very seldom.

I use a driveway alert system. It's wireless and easy to set up. Best of all its cheap, I bought mine at harbor freight for 24 bucks. It's just a beam sensor that triggers an alert tone at the receiver located near the door man. Find a good point where the group is far enough ahead that the next group isn't going to catch them, and place sensor. It great because you don't have to count on an actor (who should be focused on acting) to remember to signal the door guy.